Sheath and core textile filament



June 13, 1961 A. L. BREEN SHEATH AND CORE TEXTILE FILAMENT 2 Sheets-Sheet 1 Filed Oct. 8, 1956 INVENTOR ALVIN L. BREEN June 13, 1961 A. L. BREEN SHEATH AND CORE TEXTILE FILAMENT 2 Sheets5heet 2 Filed Oct. 8, 1956 INVENTOR ALVIN L. BREEN ATTORNEY United States Patent 2,987,797 SHEATH AND CORE TEXTILE FILAMENT Alvin L. Breen, West Chester, Pa., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Oct. 8, 1956, Ser. No. 614,640 Claims. (Cl. 28-82) This invention relates to textile fibers and more particularly to synthetic textile fibers possessing a permanent crimp and to apparatus for manufacturing them.

It has long been recognized that a strong permanent crimp in fibers, especially those used for knitting, is an essential requirement. Many methods and devices have been proposed to produce such a crimp in synthetic fibers, including a side-by-side arrangement of two dissimilar polymers, to produce a composite fiber or filament which is self-crimping upon shrinking due to a difference in shrinkage between the two components. While such fibers have some desirable uses, they are often found to have too little cohesion between the components for many purposes.

One plan for overcoming the diificulty due to separation of the fiber-forming components is an eccentric arrangement of the polymers such that one component completely surrounds the other in a sheath-core arrangement. The strength of the crimp produced by this arrangement, however, is reduced and when used to prepare a knit article the tendency to crimp is often insuflicient to overcome the restraining forces of the knit, thereby resulting in a poorly crimped fabric with little elasticity.

The sheath-core construction, however, is valuable particularly in spinning a polymer which may be difficult or impossible to spin itself or in a side-by-side arrangement, as, for example, a polyamide containing a relatively high content of amine end groups susceptible to degradation by air at the spinneret face.

It is therefore an object of the present invention to provide a composite sheath-core filament which has improved crimping properties. Another object is to provide a self-crimpable composite filament in which the core may be a compound which can not be spun alone. Still another object is to provide a spinneret which produces such filaments. Another object is to provide a process of making a sheath-core filament with a core of controlled variable shape and location. Other objects will appear as the description of the invention proceeds.

These and other objects are accomplished by an im proved spinneret more fully described hereinafter which produces a sheath-core filament having a kidney-shaped core. Variations in the shape of the core may be made by altering the rate of flow of the sheath liquid and the viscosity of the core liquid. By the term kidney-shaped (3) The area of that part of the core that is located in the half cross section containing the maximum amount of core comprises at least of the total core arrangement.

(4) The sheath essentially surrounds the core, i.e., the core is, in the main, surrounded by the sheath. While the sheath may, in some instances, fail completely to cover the core at the minimum point of the sheath dimension, it is preferred that the sheath completely envelop the core so that the sheath, at its point of minimum coverage of the core, be at least 1% of the diameter of the filament. The crimpability of the spun composite filaments is generally greatest if the minimum of sheath surrounding the core is about 1%3% of the filament diameter.

(5) The core is non-symmetrical with respect to the filament diameter enclosing the major part of the core. Thus, the periphery of the core nearest the exterior of the filament may be arcuate on an arc concentric with the center of the filament; however, the opposite side of the core may be flat, or concave or convex to a line parallel with the diameter of the filament, as it evident from the drawings.

The above definition of the term kidney-shaped core refers in the main to the filaments as-spun and before drawing, but apply equally well to the drawn filaments since the above-noted characteristics of the core persist in the same relationship in the drawn filaments.

In the drawing, FIGURE 1 is a longitudinal section of a spinneret assembly embodying the present invention. FIGURE 2 is a transverse plan section along the line 2-2 of FIGURE 1. FIGURE 3 is a transverse cross section along the line 3-3 of FIGURE 1. FIGURE 4 is a transverse cross section along the line 44 of FIGURE 1. FIGURE 1A is an enlarged section of a portion of FIGURE 1 showing more clearly a groove which alters the fiow of the liquid to the spinneret orifice to produce a kidney-shaped filament. FIGURE 5 represents a diagrammatic cross section of a prior art sheath core filament with less crimp. FIGURE 6 is a similar section of a side-by-side prior art composite filament. FIGURES 7, 8 and 9 are diagrammatic sections of filaments made according to the present invention.

In all the figures, 1 represents the front or bottom plate provided with extrusion orifices 2 and recessed at the back to form plateau-like protrusions 4. Each extrusion orifice 2 consists of an extrusion capillary 21 at the exit end and a larger counterbored portion 22 connecting the capillary with the top of the protnrsions 4. Back or top plate 7 is sealed against and spaced from the front plate by gasket 6 and shim 16, the former being ring-shaped and located near the periphery of the opposing faces of the two plates and the latter being disc-shaped and located concentric with the two plates. Relatively unrestricted region 12 between the two plates is interrupted at intervals by constricted regions 15 between the opposing face of the back plate 7 and plateau face 5 of the protrusions 4 from the front plate. The back plate is partitioned on top by outer wall 19 and inner wall 29 into cylindrical chamber 8 and central chamber 9. The cylindrical chamber communicates with the constricted regions 15 between the two plates through counterbored core feed apertures 10, consisting of terminal capillary 23 and counterbore 24, and the central chamber 9 communicates with the intervening relatively unrestricted region 12 through sheath feed apertures 11. The two plates are retained in place by screw cap 18 threaded onto the end of the backplate. The upper part of the housing (not shown) receives suitable piping or other supply means for separately supplying core and sheath forming material to .the two chambers 8 and 9, which may be provided with distribution or filtering spaces as desired. Pin 14 through cylindrical openings 25 and 26, the front and back plates respectively, near one edge of the plates ensures the desired alignment of the two plates. The back plate 7 is provided with a tapered circular groove 3 shown more clearly in FIGS. 1A and 4 which is positioned so that its shallowest depth is tangentially contiguous to each of the terminal capillaries 23. The size and shape of this groove influences the shape of the cores of the filament being extruded.

' FIGURE 2 is a transverse plan section of the front plate. Appearing in this view are eight plateau like protrusions 4, each concentric with an extrusion orifice 2 and uniformly spaced about a circle inside the outer gasket 6. FIGURE 3 shows the appearance of the back' plate sectioned as indicated on FIGURE 1, showing the concentric outer and inner walls 19'and 29, the capillaries 23 and counterbores 24 of eight core-feed apertures spaced uniformly on a circle between the two walls 19 and 29, and four sheath-feed apertures 11 located Within the central chamber 9 defined by the inner wall 29.

Operation of the described apparatus in the practice of this invention is readily understood. Separate polymers are supplied to the central and the outer cylindrical chambers 9 and 8, respectively, of the back plate; the former flows through the openings into the relatively unrestricted region 12 between back and front plates, through the relatively constricted regions 15 between the plateau face 5 and the opposing back plate face, and through the extrusion orifices 2 and capillary 21 to form the sheath of afilament while the latter passes first through the core-feed apertures 10 in the back plate 7 and directly into and through the aligned extrusion orifices 2 and extrusion capillary 21 in the front plate to form the core of the component.

The apparatus as depicted in FIGURES 1, 1A, 2, 3, and 4 but without groove 3 will afford a sheath-core yarn with concentric arrangements of the sheath and core if the orifices of the two fiber-forming components are coaxial or nearly so and the protrusions 4 are concentric the terminal capillaries 23 in the top plate. This grooved ring provides an unsymmetrical flow resistance over the plateau so'that a filament with a kidney-shaped core in cross section is produced similar to those shown in FIG- URES 7, 8, or 9 when the process of this invention is followed. a

The following examples illustrate how the apparatus operates, but no limitation is placed on the polymers used or in the method.

Example I V A spinneret assembly with 34 orifices is constructed 7 like that of the apparatus illustrated in the drawing. The

protrusions 4 on the top surface of the front plate are .0625 inch high, 0.125 inch in diameter and concentrically located about the orifices. Each extrusion orifice 2 consists of a capillary .012 inch long witha diameter of L009 'inchand a counterbore 22 of .040 inch diameter The .counterbored core-feed apertures 10 in the top plate are coaxially' locatedwith the' orifices 2 and 21in the bottom plate 1 and have an extrusion capillary 23 which is .009 inch in diameter and .012 inch long backed by a counterbore orifice 24 with a diameter of .040 inch. The constricted region 15 was made .002 inch thick by proper selection of gasket 6 and shim 16. A one-sixteenth inch wide groove 3 was cut into the face of the back plate 7 at an angle of 12 with the machined surface of the back plate in a ring around the 34 capillaries 23. The tapering edge of the groove borders the edge of the capillaries. Poly (hexamethylene adipamide) with a relative viscosity of 41 as measured on a solution of 5.5 grams of the polymer in 50 ml. of formic acid at 25 C. was fed to central chamber 9 and spun as a sheath of a composite fiber with poly(ethylene terephthalate) of relative viscosity 33 as measured on a solution of 2.150 g. of the polymer in 20 ml. of a 7/10 mixture of trichlorophenol and phenol at 25 C. as a core of the composite fiber by being fed to cylindrical chamber 8 and then spun from the above described spinneret. The pump speeds were adjusted to give a sheath/core ratio by volume of 55/45 and the two polymers were cospun at 290 C. into air at 25 C. and the resulting yarn was wound up at 1200 yards per minute as sample A of the following table.

The positions of the two polymers in the filament were reversed (with polyester as sheath and polyamide as core) using the above equipment and adjusting the sheath to core ratio to 45/55 of polyester to polyamide and yarn was spun as above. This yarn was collected as sample C of the following table.

A spinneret was constructed identical with the spinneret used above but without the plateau-like protrusions 4 and, in the absence of the groove 3, the holes for the core component 10 being ofi-set from the axis of the orifices 2 of the front plate. The same polymers as above were spun in two different arrangements under similar conditions, i.e., with the polyamide in the one case occupying the position of sheath with the polyester being the core, and with the polyester, in the other case, occupying the position of sheath with the polyamide being the core, .the former being designated as sample B of the following table, and the latter being designated as sample D of the following table. Pump speeds were adjusted in each case to yield a 40/60 volume ratio of core to sheath. This ratio had previously been determined as afiording the highest crimping force in this type of filament. Yarns of cross section similar to that of FIGURE 5 were obtained, having a circular-eccentric core.

The four yarns prepared above were drawn 300% over a hot pin at 80 C. and immediately thereafter passed over a hot plate heated to C. to stabilize the polyester component against shrinkage; they were then Wound up at 900 yards per minute (y.p.m.). Samples of the yarns were then placed in 80 C. water under varying tensions which caused the development of a spiral or helical crimp in the yarn. When crimped, the fiber had the appearance of a coiled spring with the helices reversing their direction at irregular intervals. The crimped yarns were dried under the same tensions used in crimping and lengths taken and stretched until all the crimp was straightened and the elongated length (L) of the yarn measured. The tension was released and the recovered lengthof the crimped yarn measured (L A quantity representing the tightness of the spiral crimp developed and equal to the potential elastic extensibility of the cn'mped yam termed percent crimp elongation was calculated as follows:

a The tensions used to restrain the crimping were selected as being comparable to those restraining forces found in X 1 00 2 percent crimp elongation various constructionsof knitwear and fabrics. Results of the above measurements are given in the table :below .as

percent crimp elongation from crimp tensions of varying grams per denier.

6 Example 11 Yarns with a poly(hexamcthylene adipamide) sheath The above samples had the following dimensions as measured on enlarged photographs of the cross-sections:

(1) Percent core; 45, 40, 55 and 40% respectively.

(2) The area of core located in the half cross-section containing the maximum amount of core expressed as percent of the half cross-section area; 79, 64, 91, and 64% respectively.

(3) The area of core located in the half cross-section containing the maximum amount of core expressed as percent of the total core area; 86, 82, 82, and 82% respectively.

(4) The minimum sheath thickness expressed as percent of the filament diameter: 3, 3, 1, and 3% respectively.

The improvement in crimp of the kidney-shaped core over the circular eccentric core is also obtained when the filaments in the above example are drawn over a cold pin with the omission of the hot plate treatment and crimped in boiling water.

In place of the poly(hexamethylene adipamide), also known as 66 nylon, and the poly(ethylene terephthalate) above, the following combinations of polymers are spun as composite filaments under conditions similar to above:

The copolymer poly(hexamethylene sebacamidel 66 nylon.

hexamethylene adipamide) 50/50 by weight.

After drawing and relaxing in boiling water or steam the filaments with the kidney-shaped core all show greater crimp elongation than the corresponding filaments having an eccentric circular core.

It is thus apparent that the use of the kidney-shaped core ofiers a substantial advance in the crimping force that can be developed on shrinkage of the composite fibers. This is even more striking since percent crimp elongation has previously been observed to decrease with increasing denier, and the new shape core items above are of much higher denier than their circular core counterparts. Use of the kidney-shape core thus enables a much tighter crimp to be developed and thus affords a more resilient product when such yarns are manufactured into knitwear, rugs and similar articles before being crimped, i.e., wherein the crimp must be developed against the restraining tension afforded by the fabric construction.

The improvement in crimping force provided by kidneyshaped core yarn was further observed by preparation of knit tubing of the drawn but uncrimped yarns and then crimping the yarns in the tubing with hot water. The tubings from kidney-shaped core yarns were much more elastic and resilient than those prepared from circular core yarns. Half hose were knitted from the kidneyshaped and the circular core, uncrimped composite yarns. After crimping in water at C., the kidney-shaped core test hose, particularly the polyamide sheath item, exhibited excellent crimp with subsequent elasticity comparable to the best commercial items and significantly better than the circular core test hose. In addition, since greater crimp and bulk are developed by the kidney- .shaped core items, the covering power of the hose prepared from these yarns was considerably better than those prepared from circular core yarns.

and a kidney-shaped poly(ethylene terephthalate) core similar to FIGURE 7 were spun and drawn using the plateau spinneret equipment and procedure of Example I with varying sheath/ core ratios and with the results as shown in the following table.

Percent Crimp Elongation at Drawn Crimping Tension, g.p.d. Sheath/Core, Vol. Ratio Denier The above samples have the following dimensions as measured on enlarged photographs of the cross-sections:

(1) Area of core located in half cross-section containing the maximum amount of core expressed as percent of the half cross-section area; 79, 86, and 73 respectively.

(2) Area of core located in half cross-section containing the maximum amount of core expressed as percent of the total core area; 86, 82, and 91% respectively.

(3) Minimum sheath thickness expressed as percent of the filament diameter; 3, 2, and 3% respectively.

It is seen that the maximum amount of crimping occurs when the sheath component comprises about 55% of the total filament cross section. Also, all of the crimp elongations above were superior to the best sample of a circular core yarn that could be made as shown in Example I.

Example 111 A 34 hole spinneret assembly similar to that of Example I with a groove 3 around a ring of capillaries 23 in the back plate 7 was used to spin at 800 yards y.p.m. composite filaments with a sheath of poly(hexamet-hylene adipamide), having a relative viscosity of 41, enclosing a core of poly(ethylene terephthalate) with a relative viscosity of 16. The spinning pumps were adjusted to give 51% sheath by volume and a total throughput of 16 grams per minute, which yielded after drawing a 34 filament yarn with a total denier of 140. When the constricted region 15 was .003 inch instead of .002 as in Example I, filaments of cross-section similar to those shown in FIGURES 8 and 9 were produced. The use of a substantially larger space between the plateaus and back plate gave a yarn-core cross-section approaching elliptical rather than the cross-section of FIGURES 7, 8 and 9. The use of a narrow spacing at 15 enhances the dissymrnetry of flow pattern of the sheath fluid, and a wider spacing has the reverse tendency.

The above samples made with a .003 spacing had the following drawn dimensions as measured on enlarged photographs of the cross-sections:

(1) Area of core located in half cross-section containing the maximum amount of core expressed as percent of the half cross-section area was 87.

(2) Area of core located in half cross-section containing the maximum amount of core expressed as percent of the total core was 90.

7 (3) sheath thickness expressed as percent of the filament diameter was 2%.

Example IV This example illustrates that the varying of the viscosity of the core-forming spinning component can serve to produce the kidney-shaped core of the present invention.

The equipment and procedure of Example III were used to make a series of drawn yarns comprising a poly- (hexamethylene adipamide) sheath of relative viscosity 41, and a poly(ethylene terephthalate) core of varying relative viscosity. The spacing was kept at .004 inch and the same conditions were used for each spin, with results as shown below:

Relative Viscosity Core Component Shape of Gore Sample A.. 33 Approximately ellipticaleccentrically arranged, otherwise somewhat similar to FIGURE 5.

Oblate ellipsoidal, similar to FIGURE 7.

Approximately semicircular similar to FIGURE 8.

Sample B 22 Sample O-- 19 The above samples had a minimum sheath thickness expressed as percent of the filament diameters of 15%, 7% and 5% respectively.

Example V A 34-hole spinneret similar to that shown in FIGURES 1, '1A, 2 and 3 was constructed having a tapered circular groove (3) cut into the upper plate (7) at an angle of 3 so that the shallowest portion of the groove was on a circle with a radius about greater than the radius of the circle tangent to the outermost edge of the plateaus, and the deepest part of the groove was on a circle having a radius about smaller than the radius of the circle tangent to the inner edge of the plateaus.

A copolyester was made containing 65 parts of ethylene terephthalate units and 35 parts of poly(ethylene oxide) 270% by means of drawing rolls at room temperature.

Upon boiling the drawn yarn in a tensionless condition an intense helical crimp developed. The crimp was readily pulled out under fairly low tension, but, upon the application of tension sufficiently high to draw the sheath an additional amount followed by subsequent release from this tension the yarn had a greater crimp, i.e., more crimps per inch (which was a permanent crimp), than before it was stretched.

The process of this invention enables one to control the shape and location of a core in a sheath-core filament at will. The position of the core is the result of the ,7 radial flow pattern of the sheath fluid into the orifice 22 from groove 3. If the upper and lower capillaries are coaxial and the sheath fluid flow is symmetrical, a concentric arrangement of the sheath and core is produced.

Any alteration of the symmetry of the sheath fluid'flow into the orifice causes a displacement of the core from the central position. The 'flow of the sheath fluid is V governed by the resistance caused by passing through the constricted region 15. In addition to the modification of. the 'spinneret as illustratedin the examples, and the obvious modification of using slots to-each orifice 22in the back plate rather than a continuous groove 3 to produce an asymmetrical flow, a stepped plateau with surfaces parallel to the back plate could be used, the face 5 of the plateau 4 could be cut at an angle to the back plate, the plateau could be circular but eccentrically located about the orifice 2, or any desired shape of plateau could be used to alter the flow pattern of the sheath fluid. For any given modification the rate of flow of a given fluid through a section of fixed width and length for a given pressure drop will vary as the cube of the distance between the plateau and the top plate; thus alterations as small as .001 inch in the constricted region 15 thickness will produce perceptible changes in the location of the core when an asymmetric flow already exists.

The shape of the core is effected by the viscosity of the core liquid and the radial flow of the sheath fluid, and hence the forces acting to move the core may be altered to control its position within the sheath. In a given case, as the symmetry of the sheath fluid flow is changed, the resultant forces on the core tend to flatten it on the side toward the larger flow so that in the extreme case a core concave to the direction of greater flow is produced. The opposite side of the core, i.e., that side toward the lesser flow is restricted by the force of the sheath fluid on that side so that a semi-circular shape is produced in the extreme case. The lower the viscosity of the core fluid, the more readily will its shape be changed from that as originally issued from the upper orifice.

It is obvious that a change in the volume flow ratio of core and sheath fluid will also change the shape and location of the core since the resistance of the core to movement and deformation will be a function of its volume in relation to that of the sheath volume.

Suitable pairs of components for use in this invention can be found in all groups of synthetic fiber-forming materials which have the desired physical properties and in addition possess suflicient difference in potential shrinkage between the two selected components so that a crimped fiber can be obtained. The minimum difference in potential shrinkage is preferably about 2%. The shrinkage may be developed by any known method or by the method as disclosed in my copending application Serial No. 412,781, filed February 26, 1954, and now Patent 7 No. 2,931,091, dated AprilS, 1960. r

Because of their commercial availability, ease of processing and excellent properties, the condensation polymers and copolymers, e.g., polyamides, polysulfonamides and polyesters, and particularly those that can be readily melt spun, are preferred for application in this method. Suitable polymers can be found for instance among the fiber-forming polyamides and polyesters which are described, e.g., in US. Patents 2,071,250; 2,071,253; 2,130,- 523; 2,130,948; 2,190,770; 2,465,319 and in other places. The preferred group of polyamides comprises such polymers as poly(hexamethylene-adipamide), poly(hexamethylene sebacarnide), poly(epsilon-caproamide) and the copolymers thereof. Among the polyesters that may be mentioned, besides poly(ethylene terephthalate), are the corresponding polymers containing sebacic acid, adipic acid, isophthalic acid, as well as the polyesters containing recurring units derived from glycols with more than two carbons in the chain.

Fiber-forming polysulfonamides can be produced by reacting at an interface between two immiscible phases organic sulfonic acid halides, e.g., dichlorides, which form or are contained in one phase, with primary or secondary usedin this invention comprises the polymers which contain sulfonamide groups as well as .carbonamide'groups. These polymers are conveniently produced by the same 9 method as described above, however, substituting the disulfonic acid halides by the corresponding organic monocarboxylic, sulfonic acid dihalides. The above-described interfacial polymerization methods may also be used for producing polyamides, when organic dicarboxylic acid halides are used instead of the sulfonic acid halides.

Other groups of polymers useful as components in filaments of the present invention can be found among the polyurethanes or the polyureas which may be made either by conventional methods or by the above-described interfacial methods as Well as among the polyvinyl compounds including such polymers as polyethylene, polyacrylonitrile, polyvinyl chloride, polyvinylidene chloride, and similar polymers.

The advantages of this invention accrue from the filament structure regardless of the components used or the treatment given the filaments after spinning.

The composite filaments have been produced in the examples by the melt spinning technique. Naturally, also any other spinning method like plasticized melt spinning, dry spinning, or wet spinning, can be employed successfully. In some instances, particularly when the melting behavior or the solubility of the components in a combination would not permit spinning the components by similar methods, a combination of dissimilar methods is indicated. Thus, for instance, one component, preferably the component forming the sheath, can be spun as a solution in a high boiling solvent or as a plasticizer melt, while the core-forming component is extruded as the molten polymer. In these instances, the solvents or plasticizers may be wholly or partially removed subsequently, preferably by washing them out by the help of low boiling solvents.

The product of this invention is of advantage in that it affords a higher crimping force than other eccentrically arranged sheath core yarns with the same components but retains the advantages of the sheath core structure over a side-by-side structure of the composite yarn. Some advantages of completely covering the core with a sheath are: reduction of any possible tendency of the two components to separate; the use of one component as the core that has poor spinning characteristics by itself or as the component of a side-by-side structure; the inclusion of pigments, delustrants, etc., in the core where they will not affect the surface properties, or vice versa. The fibers resulting from this invention are of great utility in knitwear, carpets, and other manner of textile fabrics in which they can be incorporatedbefore the crimping step. Alternatively, the yarn itself can be crimped to yield an elastic bulky yarn for use in various textiles.

The process of this invention is of advantage in that it afiords control of the shape and location of a core of material within a sheath of fiber-forming material. In addition to the preferred arrangement of the product of this invention, other arrangements can be made to give filaments of difiering appearance and properties.

It will be apparent that many widely difierent embodiments of this invention can be made without departing from the spirit and scope thereof, and therefore it is not intended to be limited except as indicated in the appended claims.

I claim:

1. A synthetic, self-crimping filament of fiber-forming material selected from the group consisting of condensation and vinyl addition polymers, said filament comprising a kidney-shaped core and a sheath disposed about the said core, said sheath at its point of minimum coverage of said core having a thickness of at least 1% of the diameter of the filament, the cross-sectional area of said core comprising between 40-55% inclusive of the total cross-sectional area of said filament, the crosssectional area of that part of said core that is located in the half-cross section of said filament containing the maximum amount of core material comprising at least of the half-cross-sectional area of said filament and at least of the total cross-sectional area of the core.

2. The filament of claim 1 in which the fiber-forming materials used for both sheath and core are condensation polymers.

3. The filament of claim 2 in which the condensation polymers used for both sheath and core are linear polyamides.

4. The filament of claim 2 in which the condensation polymer used for the sheath is a linear polyester and the condensation polymer used for the core is a linear polyamide.

5. A crimped filament of a fiber-forming material selected from the group consisting of condensation and vinyl addition polymers, said filament comprising a kidney-shaped core and a sheath disposed about the said core, said sheath at its point of minimum coverage of said core having a thickness of at least 1% of the diameter oi the filament, the cross-sectional area of said core comprlsing between 4055% inclusive of the total crosssectional area of said filament, the cross-sectional area of that part of said core that is located in the halfcross section of said filament containing the maximum amount of core material comprising at least 70% of the half-cross-sectional area of said filament and at least 80% of the total cross-sectional area of the core.

6. The filament of claim 5 in which the fiber-forming materials used for both sheath and core are condensation polymers.

7. The filament of claim 6 in which the condensation polymers used for both sheath and core are linear polyamides.

8. The filament of claim 6 in which the condensation polymer used for the sheath is a linear polyester, and the condensation polymer used for the core is a linear polyamide.

9. The filament of claim 6 in which the condensation polymers used for both sheath and core are linear polyesters.

10. The filament of claim 6 in which the condensation polymer used for the sheath is a linear polyamide and the condensation polymer used for the core is a linear polyester.

References Cited in the file of this patent UNITED STATES PATENTS 2,327,872 Dahle Aug. 24, 1943 2,428,046 Sisson et al. Sept. 30, 1947 2,517,694 Merion et al. Aug. 8, 1950 2,612,679 Ladisch Oct. 7, 1952 2,674,025 Ladisch Apr. 6, 1954 2,716,049 Latour Aug. 23, 1955 FOREIGN PATENTS 514,638 Great Britain Nov. 14, 1939 744,112 Germany Jan. 10, 1944 1,124,921 France July 9, 1956 

1. A SYNTHETIC, SELF-CRIMPING FILAMENT OF FIBER-FORMING MATERIAL SELECTED FROM THE GROUP CONSISTING OF CONDENSATION AND VINYL ADDITION POLYMERS, SAID FILAMENT COMPRISING A KIDNEY-SHAPED CORE AND A SHEATH DISPOSED ABOUT THE SAID CORE, SAID SHEATH AT ITS POINT OF MINIMUM COVERAGE OF SAID CORE HAVING A THICKNESS OF AT LEAST 1% OF THE DIAMETER OF THE FILAMENT, THE CROSS-SECTIONAL AREA OF SAID CORE COMPRISING BETWEEN 40-55% INCLUSIVE OF THE TOTAL CROSS-SECTIONAL AREA OF SAID FILAMENT, THE CROSSSECTIONAL AREA OF THAT PART OF SAID CORE THAT IS LOCATED IN THE HALF-CROSS SECTION OF SAID FILAMENT CONTAINING THE MAXIMUM AMOUNT OF CORE MATERIAL COMPRISING AT LEAST 70% OF THE HALF-CROSS-SECTIONAL AREA OF SAID FILAMENT AND AT LEAST 80% OF THE TOTAL CROSS-SECTIONAL AREA OF THE CORE. 